George Soros made almost $1 billion since November from bets that the yen would tumble, according to a person close to the billionaire’s $24 billion family office.

The Japanese wager helped the firm return about 10 percent last year and 5 percent so far this year, said the person, who asked not to be named because the firm is private. The yen has weakened 17 percent versus the dollar since about the start of the fourth quarter, the worst performance over a similar period since 1985.

Jan. 26 (Bloomberg) — Billionaire investor George Soros talks about the European sovereign-debt crisis, inflation risk and his Open Society Foundations. He speaks with Francine Lacqua at the World Economic Forum’s annual meeting in Davos, Switzerland. (Source: Bloomberg)

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Jan. 25 (Bloomberg) — Billionaire investor George Soros talks about Europe’s sovereign-debt crisis, European Central Bank President Mario Draghi’s policy measures and the potential impact of austerity on the region’s economies. He speaks with Bloomberg Television’s Erik Schatzker on the sidelines of the World Economic Forum’s annual meeting in Davos, Switzerland. (Source: Bloomberg)

The yen slumped and Japanese stocks rallied as Prime Minister Shinzo Abe pressed the Bank of Japan to introduce additional stimulus measures. BOJ Governor Masaaki Shirakawa and two deputies will step down next month, allowing Abe to pick leaders to implement his plan for expanded monetary easing.

Scott Bessent, chief investment officer at New York-based Soros Fund Management LLC, also has 10 percent of the firm’s internally managed portfolio betting on rising shares in Japan, said the person. Japan’s benchmark Nikkei 225 stock index has jumped about 28 percent since the end of September.

Bessent worked for Soros in 1992 when Soros and his chief strategist Stan Druckenmiller made a $10 billion bet that the Bank of England would be forced to devalue the pound. That wager netted $1 billion. At the time Soros’s Quantum fund was $3.3 billion.

Bessent left the firm in 2000, and returned to be CIO in 2011.

Michael Vachon, a spokesman for the firm, said he couldn’t comment on the trades, which were reported yesterday by the Wall Street Journal.